add (Base)ExceptionGroup (#6655)

Co-authored-by: Sebastian Rittau <srittau@rittau.biz>

Author: JelleZijlstra

stdlib/builtins.pyi

@@ -1651,3 +1651,19 @@ class ResourceWarning(Warning): ...
 
 if sys.version_info >= (3, 10):
     class EncodingWarning(Warning): ...
+
+if sys.version_info >= (3, 11):
+    _SplitCondition = type[BaseException] | tuple[type[BaseException, ...]] | Callable[[BaseException], bool]
+    class BaseExceptionGroup(BaseException):
+        def __new__(cls, __message: str, __exceptions: Sequence[BaseException]) -> BaseExceptionGroup | ExceptionGroup: ...
+        @property
+        def message(self) -> str: ...
+        @property
+        def exceptions(self) -> Sequence[BaseException]: ...
+        def subgroup(self: Self, __condition: _SplitCondition) -> Self | None: ...
+        def split(self: Self, __condition: _SplitCondition) -> tuple[Self | None, Self | None]: ...
+        def derive(self: Self, __excs: Sequence[BaseException]) -> Self: ...
+    class ExceptionGroup(BaseExceptionGroup, Exception):
+        def __new__(cls, __message: str, exceptions: Sequence[Exception]) -> ExceptionGroup: ...
+        @property
+        def exceptions(self) -> Sequence[Exception]: ...